There is a wealth of indirect evidence which supports the hypothesis that the reproductive behavior of males from a number of mammalian species is critically dependent on specialized tissue in the medial preoptic and adjacent anterior hypothalamic areas (mPOAH). This same evidence suggests that at least one of its functions is to provide a neural interface through which circulating gonadal hormones influence the transition from sexual arousal to the initiation of copulation. A third feature of this hypothesis, and one based on recent evidence, including work done in our laboratory, is that this function of the mPOA may be mediated through the medial forebrain bundle of the hypothalamus. We plan to investigate this mPOAH-MFB "system" in the following way. First, the only direct evidece for the view that the mPOAH is part of a hormone-sensitive system mediating copulation comes from the discovery that hormones, when implanted into this structure, will reinstate copulation in castrated rats. Yet several serious problems, related to questions of identification of the active steroid, localization of its site of action, and conversely, assessment of its pattern of diffusion to other brain regions, remain unresolved. We will investigate these questions by: a) identifying the mPOAH implant procedure and steroid combination (using testosterone, estradiol and dihydrotestosterone), which both maximizes the induction of copulation and minimizes surface area: b) utilize this procedure to "map" selected areas of the brain in terms of their capacity, when stimulated, to reinstate copulation; c) measure leakage of the implanted steroid into the circulation, and, more importantly, diffusion to other brain areas. Second, a direct test will be made of the hypothesis that the reinstatement of copulation in castrates by mPOAH steroid implants is dependent on a mPOAH-MFB system. If this hypothesis is correct, then the effectiveness of unilateral mPOAH implants in reinstating copulation should be impaired by ipsilateral, but not by contralateral knife cuts aimed at transecting mPOAH-MFB connections. The principal methods which will be utilized in these studies will include intracerebral hormone implant and knife cut surgery, behavioral analyses, and measurement of serum and brain tissue concentrations of steroid hormones by radioimmunoassay.